1. Field of the Invention
The present invention relates to apparatuses and methods for producing hydrogen. The embodiments described herein relate to apparatuses and methods for releasing hydrogen from chemical hydrides.
2. Description of the Related Art
Various energy sources are used to fuel today's society. Fossil fuels such as coal, oil, and gas are some of the most commonly used fuels due to the comparatively large quantities available and minimal expense required to locate, collect, and refine the fossil fuels into usable energy sources. Alternative energy sources are available. Some of the alternative energy sources are readily available; however, the cost to generate, collect, or refine the alternative energy sources traditionally outweighs the benefits gained from the alternative energy sources.
Hydrogen is a plentiful alternative energy source; however, hydrogen generally exists as a molecule combined with one or more other elements. The additional elements add mass and may prevent the hydrogen from being a usable energy source. As a result, pure hydrogen is desired for use as an energy source. Pure hydrogen comprises free hydrogen atoms or molecules comprising only hydrogen atoms. Producing pure hydrogen using conventional methods is generally cost prohibitive.
Conventionally, pure hydrogen is generated by a chemical reaction which produces hydrogen molecules. One such chemical reaction occurs between water (H2O) and chemical hydrides. Chemical hydrides are molecules comprising hydrogen and one or more alkali or alkali-earth metals. Examples of chemical hydrides include lithium hydride (LiH), lithium tetrahydridoaluminate (LiAlH4), lithium tetrahydridoborate (LiBH4), sodium hydride (NaH), sodium tetrahydridoaluminate (NaAlH4), sodium tetrahydridoborate (NaBH4), and the like. The chemical hydrides produce large quantities of pure hydrogen when reacted with water, as shown in reaction 1.NaBH4+2H2O→NaBO2+4H2  (1)
Recently, the interest in hydrogen generation from chemical hydrides has increased, because of the development of lightweight, compact Proton Exchange Membrane (PEM) fuel cells. One by-product of the PEM fuel cells is water that can be used or reused to produce pure hydrogen from chemical hydrides for fuelling the PEM fuel cell. The combination of PEM fuel cells with a chemical hydride hydrogen generator offers advantages over other energy storage devices in terms of gravimetric and volumetric energy density.
Unfortunately, the prior art has encountered unresolved problems producing pure hydrogen from chemical water/hydride reactions. Specifically, conventional systems, methods, and apparatus have not successfully controlled the chemical reaction between the water and the chemical hydride without adversely affecting the gravimetric and volumetric energy density of the overall system.
The chemical reaction between water and chemical hydrides is very severe and highly exothermic. The combination of the water and the chemical hydride must be precisely controlled to prevent a runaway reaction or an explosion. Many attempts have been made to properly control the reaction while still preserving the gravimetric and volumetric energy density provided by the chemical hydrides
One attempt to properly control the reaction involves separating water from the chemical hydride using a membrane. Generally, the membrane passes water because of a difference in water pressure across the membrane. Water pressure on the side of the membrane opposite the chemical hydride pushes the water through the membrane. Other membranes utilize a capillary action to transport water from one side of the membrane to the other. Consequently, a water supply must be provided that supplies water to the water side of the membrane to be transported by capillary action to the chemical hydride side of the membrane.
Another unfortunate side effect of such a system is that often times the chemical or anhydrous hydride will “gum” or “clump” as water is introduced. Gumming or clumping refers to the spheres or other geometric shapes formed by the chemical hydride during the reaction. Water is able to react the outer portion of the “clump” to a certain depth, however, generally large portions of the “clump” remain unreacted because water does not penetrate deeply enough. Consequently, a large percentage of the chemical hydride can remain unreacted. This is unacceptable.
Accordingly, what is needed is an improved apparatus, system, and method that overcomes the problems and disadvantages of the prior art. The apparatus, system, and method should promote a substantially complete reaction of an anhydrous hydride reactant. In particular, the apparatus, system, and method should control a chemical reaction between water and a chemical hydride using a liquid permeable material without relying on a water pressure differential across the liquid permeable material. The liquid permeable material should allow substantially only water to pass. In addition, the apparatus, system, and method should control a chemical reaction between water and a chemical hydride using a liquid permeable material that functions to maintain a thin uniform distribution of anhydrous hydride within a reaction cartridge. Such an apparatus, system, and method are herein disclosed.